1. Field of the Invention
This invention relates to a point-bonded, hydraulically jet-softened, nonwoven sheet of polyethylene film-fibril plexifilamentary strands intended for use in disposable industrial garments. More particularly, the invention concerns such a sheet that is point-bonded in such a specific way that jet-softening results in a product that is especially suited for providing comfort to the user while being a strong barrier to asbestos particles.
2. Description of the Prior Art
Spunbonded sheets of flash-spun polyethylene plexifilamentary film-fibril strands have been used in disposable industrial garments. Such sheets have been made commercially by E. I. du Pont de Nemours and Company and sold as "Tyvek" spunbonded olefin. The sheets are known for their good strength, durability, opacity and ability to act as a barrier to particulate matter as small as sub-micron size. Because of these desirable characteristics, the spunbonded sheets have been fashioned into many types of industrial garments, such as those worn by asbestos workers, as disclosed in "Protective Apparel of Du Pont TYVEK.RTM.--SAFETY YOU CAN WEAR", E-02145, (1987). However, the utility of the garments could be greatly enhanced by improvements in the spunbonded sheet from which the garment is made in order to provide a softer and more breathable garment that is more comfortable to the wearer.
Various methods have been disclosed for bonding polyethylene film-fibril sheets. For example, sheets of lightly consolidated flash-spun polyethylene film-fibril strands of the type disclosed by Steuber, U.S. Pat. No. 3,169,899 have been bonded (a) over the entire surface of the sheet, as disclosed by David, U.S. Pat. No. 3,442,740, (b) over 3 to 25% of the surface area of the sheet by passage through a loaded nip formed by a heated metal roll having 50 to 1000 hard bosses per square inch which extend from the surface of the roll to a height of at least 1.2 times the thickness of the sheet and a hard back-up roll having a Shore Durometer D hardness of at least 70, as disclosed by Miller, U.S. Pat. No. 4,152,389 and (c) over 1 to 5% of the area of the sheet by passage of the sheet through in a loaded nip formed by a heated, embossed metal roll having bosses and a soft back-up roll of a 60 to 90 Shore Durometer B hardness, as disclosed by Dempsey and Lee, U.S. Pat. No. 3,478,141. Each of the resultant bonded sheets still need improvement, especially in softness, if they are to be used for industrial garments.
Various methods have been suggested for softening bonded polyethylene film-fibril sheets. These include softening the bonded sheet by flexing the sheet under water as in a washing machine, passing the sheet over a series of rollers that have bosses that stroke the sheet, passing the sheet over a "knife edge" and the like. The use of water jets to treat point-bonded non-woven sheets has been suggested by Alexander and Baugh, U.S. Pat. No. 4,329,763. Research Disclosure, 21126, "Tyvek.RTM. softening process" (November 1981) discloses that point-bonded sheet of the type disclosed by Miller, has been softened with high energy water jets of the type disclosed by Dworjanyn, U.S. Pat. No. 3,403,862. However, improvements are still needed in such softened sheets, particularly in delamination resistance and surface durability. For example, commercially available Type 1422A "Tyvek", which has a "linen by rib" bonding pattern embossed upon it by the general method of Miller, when softened with jets of water, shows a tendency to delaminate quite readily. A sheet having its total surface bonded by the method of David, when water-jet treated has a tendency to trap water, causing large areas of delamination of the sheet.
In addition to the delamination problems associated with the water-jet-treated point-bonded sheets mentioned above, the sheets exhibit an undesirable Moire effect, when identical point-bonding patterns are employed on both sides of the sheet. The Moire problem is avoided in some point-bonded nonwovens by embossing (i.e., point-bonding) only from one side, but such sheets suffer from poor abrasion resistance and too much lint formation on at least one side.
An object of the present invention is to provide a spunbonded flash-spun polyethylene film-fibril sheet that is particularly suited for use in disposable protective garments and that would greatly alleviate the shortcomings of the above-described known sheets.